In a radio communication system, an RF receiver contains a homodyne receiver. The homodyne receiver produces an RF DC which deteriorates the quality of a signal and even causes saturation in post stage.
The value of the RF DC is influenced by such factors as, a receiving channel gain, a fading condition, a temperature and a receiving frequency (for a Frequency Modulation (FM) receiver). Any changes of these factors result in a change in the value of the RF DC. These factors affect the RF DC in different degrees, and the receiving channel gain deteriorates the RF DC most. If there is a transient in the receiving channel gain, a remarkable transient will be produced in the RF DC. Since the receiving channel gain in a communication receiver changes constantly, the RF DC also changes continuously with the receiving channel gain. When designing an RF DC canceling circuit, it is necessary to consider these factors which affect the value of the RF DC.
The RF DC harms the performance of different systems in different degrees, and the following table shows emulated performance harm that the RF DC does to a WCDMA system:
TABLE 1List of performance losses which are caused by a variety of RF DCsThe Value ofDPCHPerformance loss relativethe RF DCworking pointto the zero RF DC0−14.17 dB—25 mv−13.03 dB1.14 dB50 mv−10.78 dB3.39 dB1.0/32.0 v−12.59 dB1.58 dB1.0/64.0 v−13.00 dB1.17 dB1.0/128.0 v−13.95 dB0.22 dB1.0/256.0 v−14.05 dB0.12 dB1.0/512.0 v−14.11 dB0.06 dB1.0/1024.0 v−14.15 dB0.02 dB
As shown in table 1, the performance of the system deteriorates rapidly with an increase in the value of the RF DC.
The RF DC may be removed using a conventional method of an offset adjustment shown in FIG. 1, a conventional method of a fine gain digital cancellation loop shown in FIG. 2 respectively, or a method combined by the two.
The principle of the method of the offset adjustment shown in FIG. 1 is testing the value of the RF DC under a certain condition in advance, calling the value of the RF DC by a register under the same condition, and removing an RF DC in an input signal.
In the method of the fine gain digital cancellation loop shown in FIG. 2, the input signal is filtered using the fine gain digital cancellation loop which acts as a high-pass filter for directly filtering out RF DC offsets in the input signal.
A third method is obtained by combining the two methods. The RF DC offset in the input signal is removed by the method of the offset adjustment, i.e. the RF DC is called from the register to remove the RF DC offset in the input signal. The input signal processed by the RF DC is output to the fine gain digital cancellation loop, the input signal is high-pass filtered so as to filter the RF DC in the input signal.
Although the three conventional methods may remove the RF DC offset in the signal, they have their own shortcomings which are respectively described as follows.
When using the method of the offset adjustment, because the RF DC is affected by the receiving channel gain, the fading condition, the temperature, the receiving frequency of the FM receiver and chip technology, it is difficult to perfectly measure the value of the RF DC under a certain condition, and it is unrealistic to measure all conditions. It may be done by testing major factors which affect the RF DC, but the RF DC may not be removed effectively using the method.
When using the method of the fine gain digital cancellation loop, the narrower the filtering stop-band of the fine gain digital cancellation loop acting as a high-pass filter is, the less harm of the filtration to the performance of the system is, and the longer the digital transient response time is. For example, if there is a transient in the RF DC of the input signal, it takes a long time for the high-pass filter to effectively filter the RF DC. If the period of the transient in the RF DC of the input signal is quite short, the fine gain digital cancellation loop cannot work stably.
As for the third method, although the input signal processed by the method of the offset adjustment is high-pass filtered later, due to inherent drawbacks of the method of the fine gain digital cancellation loop, the problem that the RF DC is unable to be quickly filtered due to the transient in the RF DC of the signal, i.e. the problem existing in the method of the fine gain digital cancellation loop may still exist.
Thus, the conventional schemes may not effectively remove the RF DC offset in the signal.